1. Field of Invention
The present invention relates to an electro-optical device featuring reduced power consumption, a driving circuit and a driving method of the electro-optical device, and an electronic apparatus employing the electro-optical device as a display unit.
2. Description of the Related Art
A driving circuit of a conventional electro-optical device, such as a liquid crystal device, is constituted by a data line driving circuit, a scanning line driving circuit, etc. for supplying image signals, scanning signals, etc. at predetermined timings to data lines, scanning lines, etc. disposed in an image display region.
The data line driving circuit significantly differs in configuration, depending on whether input image signals are analog signals or digital signals. However, when display in a plurality of gray scales is performed, it is necessary to apply a voltage of an analog signal to a liquid crystal regardless of the type of an input image signal. Hence, if the input image signal is a digital signal, then the input image signal must be subjected to DA conversion so as to apply an analog signal voltage to the liquid crystal.
As a technique for the DA conversion, the PWM (Pulse Width Modulation) method is known. FIG. 12 is a block diagram showing a configuration of a liquid crystal device to which the PWM method has been applied. As shown in FIG. 12, a liquid crystal device may be constructed by a data line driving circuit 130xe2x80x2, a scanning line driving circuit 140xe2x80x2, a group of switches 150, and an image display region AA.
In the image display region AA, a plurality of scanning lines 112 are formed so that they are arranged in parallel in an X-direction, and a plurality of data lines 114 are formed in parallel in a Y-direction perpendicular thereto. Furthermore, at intersections of these scanning lines 112 and the data lines 114, thin film transistors (hereinafter referred to as xe2x80x9cTFTsxe2x80x9d) serving as switches for controlling pixels are provided.
In this example, a gate electrode of a TFT 116 is connected to the scanning lines 112, a source electrode of the TFT 116 is connected to the data lines 114, and a drain electrode of the TFT 116 is connected to a pixel electrode 118. Each of the pixels is constructed by the pixel electrode 118, a common electrode formed on an opposing substrate, and a liquid crystal sandwiched between the two electrodes; hence, the pixels are arranged in a matrix pattern in association with the intersections of the scanning lines 112 and the data lines 114. The data lines 114 oppose the common electrodes via the liquid crystal, and intersect with the scanning lines 112, so that each data line 114 is accompanied with a parasitic capacitance.
The data line driving circuit 140xe2x80x2 line-sequentially outputs selected signals corresponding to the data lines 114, based on input image data D. The period during which the selected signals are set active is decided based on input image data values to be displayed at pixels corresponding to the selected signals. Lamp wave signals LS are supplied to input terminals of switches 151 making up the group of switches 150, and output terminals thereof are connected to data lines 114, selected signals being supplied to control terminals thereof. The switches 151 are configured so that they stay ON during a period in which selected signals stay active. Hence, the lamp wave signals LS are supplied to the data lines 114 only during a period corresponding to input image data values to be displayed at pixels. As a result, the lamp wave signals are written to the parasitic capacitors of the data lines 114 only during a period corresponding to input image data values. Furthermore, the scanning line driving circuit 130xe2x80x2 generates scanning signals that become active for each horizontal scanning period and output the scanning signals to the scanning lines 112.
In the configuration described above, if a certain scanning line 112 is selected by a scanning signal, then the TFT 116 connected to that scanning line 112 turns ON in the horizontal scanning period. At this time, the lamp wave signal LS is written to the parasitic capacitor of the data line 114 only for the period corresponding an input image data value; hence, a voltage based on the input image data value is applied to the pixel electrode 118, and the applied voltage is held when the TFT 116 turns OFF. This makes it possible to display a gray scale based on a gray scale value indicated by input image data.
In the liquid crystal device set forth above, the lamp wave signals LS are written to the parasitic capacitors of the data lines 114, and the voltages of the parasitic capacitors are captured into the pixels via the TFTs 116. Therefore, the driving circuit for the lamp wave signals LS is required to have a sufficient driving capability for writing to the parasitic capacitors.
In the configuration of FIG. 12, even when the image display region AA is relatively small, a parasitic capacitance value of each of the data lines 114 is approximately 20 pF. In a liquid crystal device of a so-called XGA (1024 pixelsxc3x97768 pixels) type, 1024 data lines are provided for each color of R, G, and B, so that a total parasitic capacitance value of the data lines 114 will be approximately 61 nF. If input image data includes 6 bits, then charging must be completed in a {fraction (1/64)} H period for the capacitance of 61 nF. This means that it is necessary to use a driving circuit capable of driving a heavy load for the driving circuit of the lamp wave signals LS, presenting a problem of an increased circuit scale. Furthermore, there has been a problem in that the driving circuit consumes more power to drive a heavier load.
The present invention has been accomplished at least in view of the above, and it is an object of the present invention to at least provide an electro-optical device featuring a reduced drive load, a driving circuit thereof, and an electronic apparatus employing the electro-optical device as its display unit.
A driving method for an electro-optical device in accordance with one exemplary embodiment of the present invention is intended for driving an electro-optical device equipped with a plurality of data lines, a plurality of scanning lines, pixel electrodes corresponding to intersections of the scanning lines and the data lines, and a plurality of signal supply lines corresponding to the scanning lines. The driving method may consist of the steps of: supplying scanning signals for sequentially selecting the scanning lines; sequentially supplying reference signals to the signal supply lines synchronously when the scanning signals become active; supplying pulse width modulation signals that are active only during a period corresponding to a gray scale value indicated by image data to the data lines; and capturing the reference signals from the signal supply lines corresponding to pixels and applying them to the pixel electrodes during a period in which the scanning lines and the data lines corresponding to the pixels simultaneously become active at the pixels corresponding to the intersections of the scanning lines and the data lines, while holding voltages of the pixel electrodes during a period in which either the scanning lines or the data lines corresponding to the pixels become inactive.
According to this exemplary embodiment, as soon as scanning signals are set active, the reference signals are sequentially supplied to the signal supply lines. Hence, a load on the driving circuit that drives the reference signals will be a parasitic capacitance on a single signal supply line, so that the load can be reduced. As a result, in the step for supplying reference signals, current consumption can be considerably reduced.
Furthermore, the electro-optical device in accordance with another exemplary embodiment the present invention is assumed to have an electro-optical material sandwiched between a pair of substrates. The electro-optical device may consist of, on one of the substrates: a plurality of data lines; a plurality of scanning lines; a plurality of pixel electrodes provided in association with intersections of the scanning lines and the data lines; a plurality of signal supply lines corresponding to the scanning lines; a signal supply circuit for selecting one of the signal supply lines that has its corresponding scanning line in an active state, and supplying a reference signal to the selected signal supply line; and voltage holding circuits that are provided for the intersections of the scanning lines and the data lines, and capture the reference signals from the signal supply lines corresponding to pixels and apply them to the pixel electrodes during a period in which the scanning lines and the data lines corresponding to the pixels simultaneously become active, while they hold voltages of the pixel electrodes during a period in which one of corresponding scanning lines or data lines become inactive.
According to this exemplary embodiment, the signal supply circuit selects one having its corresponding scanning line set active from among the signal supply lines, and supplies a reference signal to the selected signal supply line. The scanning lines are adapted to be sequentially selected. Therefore, the reference signal is supplied to only one signal supply line. Thus, a load on the driving circuit for driving reference signals will be a parasitic capacitance on the single signal supply line, permitting a significant reduction in load. Moreover, the circuit configuration of the driving circuit can be made simpler, and current consumption in the driving circuit can be also considerably reduced.
Preferably, the signal supply circuit includes: a switching element provided for each of the signal supply lines, one end of the signal supply line being connected to one terminal thereof, and turning ON/OFF thereof being controlled by a signal of an corresponding scanning line; and a common signal line which is connected to the other terminal of each switching element and to which the reference signal is supplied. In this invention, the switching elements can be turned ON/OFF by the signals of the scanning lines; hence, the reference signal can be supplied only to the signal supply line corresponding to the scanning line to be selected.
Furthermore, each of the voltage holding circuits preferably includes: a first transistor element provided for each of the intersections of the scanning lines and the data lines, the first transistor having a gate electrode connected to the scanning line, and a source electrode connected to the data line; and a second transistor element provided for each of the intersections of the scanning lines and the data lines. A drain electrode of the first transistor element is connected to a gate electrode of the second transistor element. A source electrode of the second transistor element is connected to the signal supply line. A drain electrode of the second transistor element is connected to the pixel electrode.
In this exemplary embodiment, the first transistor element and the second transistor element are controlled by voltages of gate lines and the scanning lines, and the voltages of the signal supply lines are applied to pixel electrodes when the first and second transistor elements simultaneously turn ON. The reference signals are supplied to the signal supply lines when corresponding scanning lines are selected; therefore, when the first and second transistor elements simultaneously turn ON, the reference signals are applied to the pixel electrodes. With this arrangement, gray scale display based on a gray scale value of image data can be accomplished. In addition, since the data lines are connected to the source electrodes of the first transistor elements, the values of the parasitic capacitance on the data lines can be reduced. With this arrangement, the load on the driving circuit that drives the data lines can be reduced, and current consumption can be reduced.
Furthermore, each of the voltage holding circuits may include: a first transistor element provided for each of the intersections of the scanning lines and the data lines, the first transistor element having a gate electrode connected to the data line and a source electrode connected to the signal supply line; and a second transistor element provided for each of the intersections of the scanning lines and the data lines A drain electrode of the first transistor element is connected to a source electrode of the second transistor element. A gate electrode of the second transistor element is connected to the scanning line. A drain electrode of the second transistor element is connected to the pixel electrode.
In this exemplary embodiment, when the first and second transistor elements simultaneously turn ON, the voltages of the signal supply lines are applied to the pixel electrodes. The reference signals are supplied to the signal supply lines when corresponding scanning lines are selected, so that the reference signals are applied to the pixel electrodes when the first and second transistor elements simultaneously turn ON. This arrangement makes it possible to accomplish gray scale display based on a gray scale value of image data.
A driving circuit of the electro-optical device in accordance with another exemplary embodiment of the present invention may consist of: a reference signal generating circuit for generating the reference signals; a converting circuit for converting image data into line-sequential data; a pulse width modulating circuit for generating pulse width modulation signals in which pulse widths have been modulated based on data values of the line-sequential data, and outputting the pulse width modulation signals to the data lines; and a scanning line driving circuit for generating scanning signals for sequentially setting the scanning lines active, and outputting the scanning signals to the scanning lines. According to this exemplary embodiment, the reference signals are generated while line-sequentially supplying the pulse width modulation signals to the data lines and also generating scanning signals, allowing gray scale display to be accomplished by driving the electro-optical device.
Furthermore, in a driving circuit of an electro-optical device in accordance with another exemplary embodiment of the present invention, the electro-optical device is assumed to be provided with: a first transistor element that is provided for each of the intersections of the scanning lines and the data lines, the first transistor element having a gate electrode connected to the scanning line and a source electrode connected to the data line; and a second transistor element provided for each of the intersections of the scanning lines and the data lines. A drain electrode of the first transistor element is connected to a gate electrode of the second transistor element. A source electrode of the second transistor element is connected to the signal supply line. A drain electrode of the second transistor element is connected to the pixel electrode. The driving circuit may consist of: a reference signal generating circuit for generating the reference signals; a converting circuit for converting image data into line-sequential data; a pulse width modulating circuit for generating pulse width modulation signals in which pulse widths have been modulated based on data values of the line-sequential data, and outputting the pulse width modulation signals to the data lines; and a scanning line driving circuit for generating scanning signals for sequentially setting the scanning lines active, and outputting the scanning signals to the scanning lines. A low-level potential of the scanning signals is set to be higher than a low-level potential of the pulse width modulation signal by about a threshold value voltage of the second transistor.
According to this exemplary embodiment, the low-level potential of the scanning signals is set to be higher than the low-level potential of the pulse width modulation signals by about the threshold voltage of the second transistor. Hence, in a non-selection period of scanning lines, the first transistor elements corresponding to the scanning lines can be operated at a boundary between an ON state and an OFF state, making it possible to avoid a floating state of the gate electrode of the second transistor element. Therefore, the second transistor element can be securely turned OFF in the non-selection period of scanning lines.
In addition, in the driving circuit of the electro-optical device, preferably, the pulse width modulating circuit generates a pulse width modulation signal so that a high-level potential of the pulse width modulation signal is higher than a maximum potential of the reference signal by at least the threshold value voltage of the second transistor element, and the scanning line driving circuit generates the scanning signal so that a high-level potential of the scanning signal is higher than the high-level potential of the pulse width modulation signal by at least a threshold value voltage of the first transistor element. According to this exemplary embodiment, when the pulse width modulation signal is high-level, the first transistor element and the second transistor element can be securely turned ON to apply the reference signals to the pixel electrodes.
Furthermore, the reference signals are preferably lamp wave signals. However, when gamma correction is performed by using the reference signals, reference signals following a gamma correction curve may be used.
Furthermore, the driving circuit described above may be formed on one of the two substrates of the electro-optical device. In this case, the transistor elements making up the driving circuit may be fabricated using the same manufacturing process for the first and second transistor elements thereby to reduce manufacturing cost.
In addition, an electronic device in accordance with various exemplary embodiments of the present invention may consist of the foregoing electro-optical device, so that power consumption can be reduced.